The mechanism of ribosome-catalyzed, messenger RNA-directed protein synthesis is fundamentally the same in all organisms, and it is important that it be fully understood for many reasons. First, protein synthesis is a major metabolic activity in all organisms. Second, novel RNA-dependent enzymology may be involved. Third, because so many antibiotics target bacterial ribosomes, an understanding of the details of the ribosomal phase of translation may have significant clinical implications. Since lack of atomic resolution information about ribosome conformation has serious limited progress in this area of inquiry for many years. The focus of the work proposed here is the determination of the structure of the ribosome and the complexes it forms with antibiotics and the macromolecules with which it interacts by X-ray crystallography. Four projects will be undertaken all in collaboration with R.A. Steitz to a greater or lesser extent. First, the effort to solve crystal structure of the large ribosomal subunit from Haloarcula marismortui, which is already well underway, will be brought to a conclusion. The crystals available diffract past A resolution, and interpretable electron density maps of the structure can be computed today to 5 A resolution. Second, crystals will be prepared of domains of the small ribosomal subunit, in hopes of obtaining information about the conformation of that subunit at resolutions significantly higher than those accessible using the crystals of intact small subunits currently available. Third, a program will be instituted the objectives of which is the determination of the crystal structures of isolated proteins from the large ribosomal subunit of H. marismortui, or of other archael species, to facilitate the interpretation of the electron density maps of the H. maris mortui 50 ribosomal subunit that are becoming available. Fourth, crystals will be prepared of ribosomes from eukaryotic species, with the ribosomes form yeast being the first objective.